1. Field of Invention
The disclosure relates to an output amplifier. More particularly, the disclosure relates to the output amplifier of a source driver for driving a display panel.
2. Description of Related Art
In general, a liquid crystal display usually includes a source driver for driving source lines (or column lines) in order to drive the LCD panel. The source driver provides source driving signals or input pixel signals to each of the source lines in order to indicate the color/data in the respective source lines, and displays a picture in an LCD.
Since most source drivers are used in portable devices such as the laptop computers or the notebooks, and these portable devices usually work in low analog power, therefore, reduction in power consumption becomes a very important issue. However, source driver having the output amplifier manufactured in high voltage process (ex. 12V) can hardly work in low analog power due to body effect of the transistor employed in the out amplifier.
The body effect is the threshold voltage variation raised by the change in the source-bulk (body) voltage, in other words, if the source and the body of the transistor are not connected together, the body effect problem appears, and the equivalent resistance of the transistor is raised. The body effect can be approximated by the following equation:VTN=VT0+γ(√{square root over (VSB+2φ)}−√{square root over (2φ)}),
where VSB is the voltage drop between the source and the body of the transistor, VT0 is threshold voltage when VSB is zero, VTN is the result threshold voltage with substrate bias present, γ is the body effect parameter, and 2φ is the surface potential parameter. Because the body can be operated as a second gate, and is sometimes referred to as the “back gate”, the body effect is sometimes called the “back-gate effect”.
Due to the body effect, the driving ability of the output amplifier is weakened, and the driving ability of the output amplifier is even worse when the analog power is reduced, which might not be able to drive the pixel circuit.
Therefore, there is a need for a new output amplifier which can preserve the driving ability to drive the pixel circuit.